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The effect of the bite angle on regioselectivity in the rhodium-catalyzed hydroformylation reaction was studied with a series of bidentate diphosphines based on xanthene-like backbones as ligands. The bite angles of these ligands are fine-tuned by subtle alterations of the backbone of the ligands. When the bridge (X) in the 10-position of xanthene is varied, the bite angle as calculated from molecular mechanics increases stepwise from 102 to 131", whereas the changes in steric bulk and electronic effects are virtually absent for the following ligands: bis(2-(diphenylphosphino)phenyl) ether (DPEphos, 11, X = H, H; 4,6-bis(dipheny1phosphino)-10,lO-dimethylphenoxasilin (Sixantphos, 2), X = Si(CH&; 2,8-dimethyl-4,6bis(dipheny1phosphino)phenoxathiin (Thixantphos, 3), X = S; 9,9-dimethyl-4,6-bis(dipheny1phosphino)xanthene (Xantphos, 4), X = C(CH&; 4,6-bis(diphenylphosphino)dibenzofuran (DBFphos, 51, X = bond. In the hydroformylation of l-octene the regioselectivity increased regularly with increasing bite angle: a t 40 "C up to 98.3% n-aldehyde was obtained with Xantphos, without isomerization or hydrogenation of l-octene. DBFphos does not form chelates, and consequently no increased selectivity was observed. The selectivity of the catalyst was almost unaffected by raising of the temperature to 80 "C, resulting in a higher turnover frequency ( to0 with a constant selectivity: 97.7% n-aldehyde, 0.5% isomerization, and a tof value of 800 mol (mol of Rh)-l h-l. Xantphos induces the highest selectivity for the formation of the linear aldehyde reported for diphosphines in the hydroformylation of l-alkenes until now. The complexes (diphosphine)Rh(H)(CO)(PPh3) and (diphosphinelRh(H)(CO)z were prepared and identified with lH, 31P, and 13C NMR. The enhanced selectivity t o the linear aldehyde was also observed for styrene (70% n-aldehyde with xantphos compared to 11% with triphenylphosphine). An X-ray crystal structure of the Xantphos ligand is presented (orthorhombic, space group Pbnm, with a = 8.7678(8) A, b = 18.967(1) A, c = 19.181(1) A, V = 3189.8(4) Hi3, and 2 = 4).

organometallics

New Diphosphine Ligands Based on Heterocyclic Aromatics Inducing Very High Regioselectivity in Rhodium-Catalyzed Hydroformylation: Effect of the Bite Angle